Now, she is trying to help her daughter deal with some of the severe morning sickness that has been plaguing her for weeks.

"I have said to Jill, 'When you feel sick and so tired, remember you are growing another person inside of you and this is a good reason to be sick,' " Michelle says. "For me, I would try to sit down and rest whenever I was sick while pregnant. You have to do what you have to do. The end result is so worth it."

Jill's father, Jim Bob, 49, says though he was shocked about the news, he knows the new couple will be wonderful parents.

"Jill has been such a wonderful sister to all of her brothers and sisters," he tells PEOPLE. "She always makes activities for the kids, looks for ways to make a dull project fun. And Derick is going to be a great father."

When Derick shared the news with his own family – including his mom, Cathy, 57, who is now cancer-free after battling through intensive chemotherapy during preparations for Jill and Derick's wedding – the joy continued to spread.

"I was unable to be involved in any of the wedding preparations for Jill and Derick," she says. "And so I am really looking forward to helping them get ready for the baby!"

The new grandma-to-be, who is adopted, is excited that along with her two sons – Derick and his younger brother, Daniel – she will now have three blood relatives in the world. That's a huge contrast to the hundreds of Duggar extended family members, and Cathy is already doing some shopping in anticipation of her new grandchild.

"Oklahoma State is my alma mater," she says. "This year, they came out with a T-shirt that says 'Real grandmas don't wear pink, they wear orange.' I plan to get one of those. I am looking forward to every aspect of them having this baby."

One thing is certain – the couple will have no shortage of babysitters when their child is born.

"I am up for babysitting if they need help," says Jill's younger brother Jedidiah, 15. "I am so excited for Jill and Derick and I know they are going to make wonderful parents. Jill has always wanted to be a mom. She is perfect for it. Derick is a wonderful guy, he will be a good dad. He will be amazing."

For much more on the couple's pregnancy and plans for a supersize family of their own – and exclusive details on her sister Jessa's romantic engagement to Ben Seewald – pick up this week's issue of PEOPLE, on newsstands Friday